I just borrowed some ebooks from my library using Adobe Digital Editions. I have an Adobe ID and I authorized my nook. I was able to download the book, but it will not open on my nook. I get a message that says "user not activated". Does anyone know what this means and how I can fix it? I did not have any trouble reading the free public domain books my library offers, just the borrowed books.

You need to:
1) hook your nook up to the computer and delete the ENTIRE folder named "adobe digital editions" through My Computer
2) restart your nook (totally off and then totally on)
3) hook the nook back up to the computer and open ADE. It will ask to authorize device, do so, and then drag the books and you'll be ready to go.

I followed the steps described by jedifarfy exactly and still was not prompted to authorize nook. Can you suggest anything else? I do not have the folder .adobe-digital-editions on my nook. I am using a Mac if that changes anything.
Thanks.

I followed the steps described by jedifarfy exactly and still was not prompted to authorize nook. Can you suggest anything else? I do not have the folder .adobe-digital-editions on my nook. I am using a Mac if that changes anything.
Thanks.

Any filename that starts with a period is "hidden" by the Mac OS. That's why you can't see it.

Edit: Open Terminal (under Utilities) on your Mac and type in the following (as is), or copy and paste: